The invention relates to an apparatus and method using electrical current to measure certain properties of a target surface having a substrate with an insulative coating thereon, and is particularly useful for measuring properties of naturally occurring tissue, and more particularly for measuring properties of the stratum corneum.
For years those of ordinary skill in the art have attempted to make measurements of various properties of target surfaces. As used herein, a target surface comprises a substrate having one or more aesthetically, electrically or otherwise functionally distinguishable layers superimposed thereon. The substrate may be disposed between other components of system under consideration or may be the inner component of the system under consideration. At least one superimposed layer is disposed on the substrate and may provide a protective function or other functionality. The protective layer under consideration is the outwardly facing protective layer, which is exposed when the target surface is in use. To be distinguishable and measurable according to the present invention, it is only necessary that the superimposed layer of the target surface have a greater electrical resistance than the substrate and is relatively thin, as discussed below.
The measurements of the present invention are applicable to target surfaces comprising natural tissue. As used herein, natural tissue comprises tissue originating from the animal, plant and mineral kingdoms.
A target surface of particular interest is the human skin. The measurement of various properties of the human skin is important, as the stratum corneum, i.e. the outermost layer of the human skin, provides several functions. For example, the stratum corneum provides a barrier between the inside of the body and the outside world. Maintaining this barrier is important to preventing the intrusion of chemicals, bacteria and viruses into the body. The stratum corneum also regulates the evacuation of moisture from the body. However, measurement of the barrier properties of the human skin must be sanitary, noninvasive and usable with people of differing ages and health conditions.
The barrier properties of the skin are typically measured by transepidermal water loss. However, this measurement relies upon complex equipment and can be influenced by unrelated health and environmental factors, such as humidity, alcohol intake, Parkinson""s disease and certain headaches. Transepidermal water loss is discussed in more detail in Chap. 9 of the Handbook of Non-invasive Methods and the Skin, editors Serup and Jemec, published by CRC Press, copyright 1995, incorporated herein by reference.
Likewise, other target surfaces of interest include the skin of pets, such as dogs and cats. The condition of the pet""s skin can indicate the general overall health and nutrition of the pet. This feature is particularly important, as the pet cannot directly communicate its health concerns or needs.
U.S. Pat. No. 5,738,107 iss. Apr. 14, 1998 to Martinsen et al., discloses a technique for measuring the moisture content of the skin by placing three electrodes in contact with the skin. A voltage having a frequency less than 50 kHz is applied to the electrodes. The susceptance is then measured under the electrodes. Yet other attempts in the art directly measure phase angle. Illustrative are U.S. Pat. No. 3,665,302 iss. May 23, 1972 to Lees et al.; U.S. Pat. No. 4,758,778 iss. Jul. 19, 1988 to Kristinsson; and GB 22988923 pub. Sep. 18, 1996. However, each of these attempts relies upon AC and complex measurements of phase angle.
U.S. Pat. No. 6,085,115 iss. Jul. 4, 2000 to Weaver et al, discloses a a biopotential measurement that decreases the resistance of the skin to electrical fields. The topical application of a resistance-decreasing agent is also taught in order to diminish unwanted voltages, which compete with the biopotential measurements. The biopotential measurement is made at a skin surface site, which is electroporated through strong electric field pulses. In this method, electroporation is used as an auxiliary means to improve quality of the biopotential measurements and does not indicate properties of the skin.
WO 001301 A1, published Jul. 6, 1999 to Szopinski teaches measurement of skin resistance/impedance. Both alternating and direct current may be used to yield AC impedance or DC resistance, respectively. However, each of these teachings relies upon a resistance/impedance measurement to determine skin properties. Resistance measurements are dependent on the value of applied voltage and a number of other factors and, hence, may have low accuracy. Impedance measurement methods have better accuracy but they require complex equipment due to high frequencies involved.
U.S. Pat. No. 5,239,258 iss. Aug. 24, 1993 to Kauffman teaches a method of measuring the freshness of fuels, oils, and food products by, inter alia, dissolving a sample of the material to be tested in a solvent. The solvent is selected to dissolve both the material and its oxidation products. This procedure is infeasible when one wishes to preserve the integrity of the sample.
None of the attempts in the prior art known to the inventor teach a method of measuring properties of the skin utilizing relatively high voltages and which rely upon or cause ion transfer through the stratum corneum.
Furthermore, the method and apparatus of the present invention may be broadly applied to measurements of the barrier properties of many natural coatings disposed on many natural substrates, particularly those that are electrically conductive. For example, the present invention may be used to measure certain properties of any thin insulating layer deposited on the surface of a conductive natural substrate, e.g. the permeability of the waxy layer of a plant leaf, barrier properties of a plant root, the effectiveness of chemical coatings on plants, the grade of leather, waterproof resistance of cloth, residual dust/dirt on conductive surfaces, bacteriological, viral, mold or other contamination on conductive surfaces. For example, the invention may be used to test food products, e.g. as the outer layers of vegetables such as potatoes, carrots, apples and oranges.
In one embodiment the invention comprises a method for measuring the barrier properties of a target surface comprising natural tissue. The method comprises the steps of providing a probe. The probe has a pair of spaced apart electrodes in electrical communication with each other, providing a voltage generator, capable of supplying an increasing voltage between said electrodes, and providing a voltage meter capable of indicating the voltage between said electrodes. The electrodes are placed in contact with the skin of a test subject. An increasing voltage is supplied from the voltage generator to the electrodes until current between said electrodes reaches a predetermined value. The voltage, which occurs when said current reaches the predetermined value, is noted.
In another embodiment the invention comprises a device for measuring the barrier properties of a target surface comprising natural tissue. The device comprises a probe. The probe has a pair of spaced apart electrodes in electrical communication with each other. The electrodes are contactable with the skin of a subject. The device also comprises a voltage generator, capable of supplying an increasing voltage between the electrodes, and a voltage meter capable of indicating the voltage between the electrodes. The voltage meter indicates the voltage between said electrodes when current therebetween reaches a predetermined value.
One of skill will recognize the aforementioned method and device have applicability to measurements of the target surface moisture content, barrier properties and other properties as well. For example, measurements of other barriers, coatings, and laminae on various substrates may be made according to the present invention.